1. Field of the Invention
The present invention relates in general to valve actuation devices of an internal combustion engine, and more particularly to the valve actuation devices of a valve lift switching type that switches a valve lift characteristic of intake and/or exhaust valves in accordance with an operation condition of the engine.
2. Description of the Related Art
Hitherto, for achieving a reduced fuel consumption in a low and middle speed operation and an improved output torque in a high speed operation, various valve actuation devices have been proposed and put into practical use in the field of internal combustion engines for wheeled motor vehicles. Some of them are of a valve lift switching type that switches the valve lift characteristic of intake and/or exhaust valves in accordance with an operation condition of the engine.
In order to clarify the task of the present invention, one known valve actuation device of such type will be briefly described before describing the detail of the invention, which is shown in Japanese Laid-open Patent Application (Tokkaihei) 5-171909.
In the valve actuation device of the publication, a lower speed rocker arm having one end contactable with an intake valve is pivotally held by a lower speed rocker shaft and a higher speed rocker arm is arranged beside the lower speed rocker arm and pivotally held by a higher speed rocker shaft. Lower and higher speed cams are in contact with the lower and higher speed rocker arms respectively. The higher speed cam is so shaped as to cause the intake valve to have a higher lift degree and a greater working angle than those caused by the lower speed cam.
A hydraulically actuated coupling mechanism is incorporated with the lower and higher speed rocker arms to selectively couple and uncouple the same.
Under operation of the engine, a control unit controls or actuates the coupling mechanism with a hydraulic power in accordance with an operation condition of the engine. That is, when the engine is subjected to a lower speed operation, the controller controls the coupling mechanism to uncouple the two rocker arms thereby activating the lower speed rocker arm and thus causing the intake valve to have a lower lift degree suitable for the lower speed operation. While, when the engine is subjected to a higher speed operation, the controller controls the coupling mechanism to couple the two rocker arms thereby activating the higher speed rocker arm and thus causing the intake valve to have a higher lift degree suitable for the higher speed operation. More specifically, in the lower speed operation, the intake valve lift degree is controlled relatively small and the valve close timing of the intake valve is made before the bottom dead center (BDC) of the piston, so that undesired pumping loss and mechanical friction are reduced and thus the fuel consumption of the engine is improved. While, in the higher speed operation, the intake valve lift degree is controlled relatively large and the valve open timing of the intake valve is advanced, so that intake air charging is increased and thus satisfied output power of the engine is obtained.